1. Field of the Invention
The present invention relates to a substrate on which optical components used in optical fiber communication for transmitting high-definition image signals or the like are mounted, and a method of producing the substrate. More particularly, the invention relates to an optical component mounting substrate which is used in an input/output coupler for connecting optical components with an optical fiber that functions as a transmission line.
2. Description of the Related Art
Optical components used in optical fiber communication, such as optical switches, branching filters and polarizing splitters, are usually mounted on a substrate for the purpose of integrating and modularizing components comprising a communication system. Such a substrate is called "an optical component mounting substrate" here.
A typical configuration of an optical component mounting substrate 80 is shown in FIG. 8. On a surface of a base material 81, a V-shaped groove 82 and a four-cornered groove 83 which crosses the V-shaped groove 82 are formed. The V-shaped groove 82 holds an optical fiber (not shown here), called "an optical fiber holding groove" here. The four-cornered groove 83 holds an optical component that will be inserted therein, and be called "an optical component insertion groove" here.
A sheath of an intermediate portion of the optical fiber is removed and a clad in the portion is exposed. This exposed portion is called "a clad-exposed portion". Then the optical fiber is placed linearly along the optical fiber holding groove 82 in such a way that the clad-exposed portion is located in the optical component insertion groove 83. Therefore, the clad-exposed portion of the optical fiber can be optically connected to the optical component inserted and fixed in the groove.
The conventional substrate 80 of the prior art is made of silicon or ceramic. The base material 81 is subjected to manufacturing processes such as etching or dicing saw grinding, and then a precision finishing process is conducted so as to form the V-shaped groove 82 and the four-cornered groove 83 as shown in FIG. 8 (e.g., Japanese Laid-Open Patent Publication Nos. 4-352109 and 5-134146).
In another prior art, a substrate made of plastics is produced by an injection molding method so that grooves can be efficiently formed on its surface (e.g., Japanese Laid-Open Patent Publication No. 5-19131).
From the view point of modularization and integration of optical components for optical fiber communication, it is necessary to assemble optical waveguides and optical components into an optical fiber system.
Most of optical components are optical waveguide devices. The diameter of those waveguides is typically of the order of several microns. On the other hand, optical fibers which are mainly used in optical fiber communication are at present single-mode fibers, which have a core of the order of several microns in diameter and this core functions as an optical transmission line.
Those components and optical fibers must be connected optically to each other to achieve communication. Since the misalignment of cores in an optical coupler causes coupling losses to increase, it is necessary to connect optical waveguides with cores of optical fibers, both of several microns in diameter, with a positional accuracy of 0.5 .mu.m or less, and with excellent reliability, and moreover, at a low cost.
Sometimes, it is necessary to have a plurality of connections of optical fibers in close proximity to each other. In other cases, V-shaped grooves at input must be formed at different intervals from the output. Furthermore, it is necessary to have optical components and electrical conductors fabricated on the same substrate at the same time.
However, conventional manufacturing methods of optical component mounting substrates of the prior art as mentioned above have many disadvantages.
When a conventional grinding process is conducted for forming grooves on a surface of a substrate, the grooves must be ground one by one. Consequently, it takes a lot of time so production cost increases.
When the number of grooves to be formed is large, a very large number of grinding steps must be conducted. Moreover, it is very difficult to grind different intervals of the grooves side by side.
With the grinding process, intervals of the grooves is limited to a certain level by the operational restriction of the grinding machine used in the process. Therefore, when plural optical fibers are to be connected in close proximity to each other, V-shaped grooves which are to hold the fibers must be ground not in parallel but in a radial manner.
In this case, grinding the grooves in a radial manner with a dicing saw may be conducted instead. That is, however, cumbersome and gives poor fabrication result in which the incident angle of each optical fiber holding groove is not constant. This is because a dicing saw has poor accuracy of rotation angle. Therefore, the coupling loss in that case becomes large.
A photolithograph technique and an etching process can be applied for groove forming. A desired pattern of grooves is formed on a silicon substrate with a photolithograph technique, and then an etching process is conducted with the pattern as a mask in order to form the actual grooves on the substrate surface. However, having grooves of different depth side by side is difficult to create in this manner because of etching characteristics.
In a silicon substrate or a ceramic substrate, it is difficult to integrally form optical waveguides with the grooves in one process. On the contrary, such an integral formation can be conducted with a plastic injection method. Unfortunately, it is difficult to form optical fiber holding grooves and optical waveguides with a positional accuracy of 1 .mu.m or less in this manner, because plastics have high water absorption and poor heat-resistance, and as a consequence can be easily distorted.